1. Field of the Invention
The present invention relates to a light source module. More particularly, the present invention relates to a light source module for a scanning projection apparatus.
2. Description of Related Art
Referring to FIGS. 1A and 1B, a conventional scanning projection apparatus 100 includes a light source module 110 and a mirror 120. According to an input signal of the image to be displayed, the scanning projection apparatus 100 makes a light beam 112 emitted by the light source module 110 incident into the mirror 120 which swings in two axial directions, and then the light beam 112 is deflected by the mirror 120, so as to sequentially project the light beam 112 to different positions on a screen 30 to form an image.
In particular, if the resolution of the image projected by the scanning projection apparatus 100 is M×N (i.e., having M×N pixels), the mirror 120 sequentially projects the light beam 112 to positions P(1,1), P(1,2), . . . , P(1,N), . . . , P(2,1), P(2,2), . . . , P(M,N−1), and P(M,N) on the screen 30. Moreover, if the refresh frequency of the image is 60 Hz, the light source module 110 must emit (M×N) light beams 112 with different light intensities in 1/60 second.
Referring to FIG. 2, the conventional light source module 110 includes a red laser 114r, a green laser 114g, a blue laser 114b, and an X-prism 116. The red laser 114r, the green laser 114g, and the blue laser 114b respectively provide a red light 112r, a green light 112g, and a blue light 112b. Moreover, a light splitting surface 116a of the X-prism 114 reflects the blue light 112b, and lets the green light 112g and the red light 112r pass there through. A light splitting surface 116b of the X-prism 114 reflects the red light 112r, and lets the green light 112g and the blue light 112b pass there through, so as to combine the red light 112r, the green light 112g, and the blue light 112b into the light beam 112.
In view of the above, if the scanning projection apparatus 100 needs to employ the modulation of the laser intensity in the gray-scale application, the light source module 110 must sequentially emit (M×N) light beams 112 in 1/60 second according to the input signal of the image to be displayed, so the light intensity modulation frequency of the red laser 114r, the green laser 114g, and the blue laser 114b must be 60×(M×N) Hz. A resolution of 1024×768 is taken for example; the light intensity modulation frequency must exceed 47 MHz. Currently, the light intensity of the red laser 114r and the green laser 114g are adjusted by modulating the driving current at a frequency of larger than 47 MHz. However, the light intensity modulation frequency of the green laser 114g cannot reach 47 MHz. Therefore, in the conventional art, a light intensity modulator 118 is added between the green laser 114g and the X-prism 116, so as to modulate the light intensity of the green light 112g by way of external modulation.
In the conventional art, a common way of external modulation is an electro-optical modulation (EOM) of using an electro-optical crystal as the light intensity modulator 118, or an acoustic-optical modulation (AOM) of using an acoustic-optical crystal as the light intensity modulator 118. However, as it is difficult and expensive to fabricate the electro-optical crystal and the acoustic-optical crystal, the cost of the light source module 100 is increased. Furthermore, the process of employing the EOM or AOM consumes substantial amount of power (about 1 watt), which is not suitable for being applied to a scanning projection apparatus powered by batteries. In addition, the green light 112g must pass through the electro-optical crystal or the acoustic-optical crystal as a whole, which causes an additional energy loss.